Maria,
I think you are unduly pessimistic. Olives should come into production well before 15 - 20 if they are fertilised and looked after. In this part of Italy we expect to wait 8 to 10 years but that is because we have frequent winter frosts that scorch the growing tips of the young trees and slow growth.

In Australia where the winters are much milder I have seen trees in full production at 8 years (with irrigation)

The climate still worries me. You talk about wet and dry seasons but the Mediterranean climate where olives thrive is winter - wet and summer dry. Your climate is the other way round I believe. The summer rainfall causes problems for olives but they are not necessarily insurmountable.

One lesson to learn from the Australian experience is not to plant a mish-mash of cultivars from all over the world. Olives need cross-pollination for optimum yield and the cross-fertilisation of cultivars from different countries is not known - although Australian researchers are trying to catch up on this work.

Cheers Brian Chatterton